chore: remove graduated preview header

Author: dessant

src/index.js

@@ -106,18 +106,13 @@ class App {
  }
 
  core.debug(`Locking (${type}: ${issue.issue_number})`);
- let params;
+
+ const params = {...issue};
+
  if (lockReason) {
- params = {
- ...issue,
- lock_reason: lockReason,
- headers: {
- accept: 'application/vnd.github.sailor-v-preview+json'
- }
- };
- } else {
- params = issue;
+ params.lock_reason = lockReason;
  }
+
  await this.client.rest.issues.lock(params);
 
  threads.push(issue);